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Sensors and ActuatorsControl systems and Computer Networks
Dr Alun Moon
Lecture 4.2
Dr Alun Moon Sensors and Actuators Lecture 4.2 1 / 18
Layers from Program to Physical
CPUlogic 0,1
Voltages0V–3.3V
SignalsADC,DAC
Physical WorldReal stuff
Sensor Actuator
MeasurePhysicalQuantities
MakePhysical
things happen
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Layers from Program to Physical
CPUlogic 0,1
Voltages0V–3.3V
SignalsADC,DAC
Physical WorldReal stuff
Sensor Actuator
MeasurePhysicalQuantities
MakePhysical
things happen
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Sensors
I Measure some value in the environmentI allow sampling of process variableI convert changes in the physical process into electrical signals
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Actuators
I Change something in the environmentI Under computer controlI provide feedback to physical process to maintain some conditionI convert electrical signals into physical changes
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Sensor ExamplesPhysical quantities measurable
I TemperatureI PressureI Distance and DisplacementI VelocityI AccelerationI Fluid FlowI Light intensity
I VoltageI CurrentI ResistanceI ForceI Liquid levelsI TorqueI pH
And many more. . .
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Part I
Sensors
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SwitchesBinary Devices
I Position – close contacts – confirm position of mechanismI Human hands – keyboardsI Conditions – pressure, temperature (thermostat)I Medium
• electrical• pneumatic• hydraulic
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Pressure measurement
I Pressure bends an elastic element such as a diaphragm, tube, bellowsor piston
I The displacement in turn moves a needle, change an electricalimpedance or resistance
I Piezoelectric pressure sensorsI Rapid changes in pressure are difficult to measure – why?I High pressure transducers are costly
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Temperature
I Expansion of solids, liquids or gasesI Pressure or movement changes can be measuredI Thermocouples – junction between dissimilar metals – generate small
voltagesI Other solid state devices are available such as thermistors
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Acceleration
I Typically MEMS devicesI Silicon chip scale mechanismI Moving mass
• flexes strain gauge – measure resistance• changes air gap – measure capacitance• piezoelectric – strain crystal, generates voltage
FXOS8700CQ All information provided in this document is subject to legal disclaimers. © NXP B.V. 2017. All rights reserved.
Data sheet: Technical data Rev. 8.0 — 25 April 2017 7 of 113
NXP Semiconductors FXOS8700CQ6-axis sensor with integrated linear accelerometer and magnetometer
7.1 Orientation
8. Terminology
8.1 Sensitivity
Sensitivity is represented in mg/LSB for the accelerometer and µT/LSB for the magnetometer. The magnetometer sensitivity is fixed at 0.1 µT/LSB. The accelerometer sensitivity changes with the full-scale range selected by the user. Accelerometer sensitivity is 0.244 mg/LSB in 2 g mode, 0.488 mg/LSB in 4 g mode, and 0.976 mg/LSB in 8 g mode.
8.2 Zero-g and zero-flux offset
For the accelerometer, zero-g offset describes the deviation of the output values from the ideal values when the sensor is stationary. With an accelerometer stationary on a level horizontal surface, the ideal output is 0 g for the X and Y axes, and 1 g for the Z-axis. The
Fig 4. Product orientation and axis orientation
Top view
Pin 1
Side view
TOP
BOTTOM
+Ax, +Mx
+Ay, +My
+Az, +Mz
1
Top view
Xout @ 0 g
Magnetic FieldEarth Gravity
Maximum My
Minimum MxMaximum Mx
Minimum My
Maximum Mz
Minimum Mz
Yout @ –1 gZout @ 0 g
Xout @ 1 gYout @ 0 gZout @ 0 g
Xout @ –1 gYout @ 0 gZout @ 0 g
Xout @ 0 gYout @ 1 gZout @ 0 g
Xout @ 0 gYout @ 0 gZout @ 1 g
Xout @ 0 gYout @ 0 gZout @ –1 g
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Magnetometer
I Senses magnetic fields• 1D• 2D
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Part II
Actuators
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DC Motors
DC motors are employed very widely in industry, appliances, automobiles,etc.
I Used to provide continuous rotation or no rotation - positionI Inexpensive and efficientI Can use PWM for speed control - noisyI Geared for more torque
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Stepper Motors
I Digitally controlledI Discrete positioningI Useful where accurate control is requiredI Lower torque than DCI Pulses cause the motor to rotate in steps - perhaps 1.8◦ per pulseI Positional feedback is not required (unless the motor slips)
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RC Servos
Used in Radio Control applications, and widely used in small scale systemsI 3-pin connections Signal, Voltage, GroundI PWM controlled
• Pulse width determines position• 1.5ms “neutral” position• 1.0ms sets to 0◦
• 2.0ms sets to 90◦
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Part III
Limitations
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Limitations
Devices have some built in limitations on how they operateAccuracy the total of all deviations between a measured value and the
actual value - sum of non-linearity, repeatability andhysteresis.
Non-linearity the maximum difference in measured value or output from astraight line between calibration points
Repeatability the max difference in a measured value or output when a setpoint is approached multiple times from above or below
Hysteresis the max difference in measured value or output when a setvalue is approached from above, and then below the value.
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Accuracy and Precision
Accuracy How close to the actual value (related to repeatability)Precision How fine a measurement can you make – how close can two
values be and till be distinguished.
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